Understanding superbursts
Jean in ’t Zand
SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, the Netherlands
E-mail: jeanz@sron.nl
Abstract
Superbursts were discovered at the beginning of this millennium. Just like type-I X-ray bursts, they
are thought to be due to thermonuclear shell flashes on neutron stars, only igniting much deeper. With
respect to type-I bursts, they last 103 times longer, are 103 as rare, ignite 103 times deeper (in column
depth) and are thought to be fueled by carbon instead of hydrogen and helium. Observationally, they aresometimes hard to distinguish from intermediate duration bursts which are due to pure helium flashes oncold neutron stars. So far, 26 superbursts have been detected from 15 neutron stars in low-mass X-ray
binaries that also exhibit type-I bursts. They are very difficult to catch and only 2 have been measured
with highly sensitive instrumentation. Superbursts are sensitive probes of the neutron star crust and theaccretion disk. The superburst phenomenon is not fully understood. Questions remain about the natureof the fuel, the collection of that fuel and the ignition conditions. The current state of affairs is reviewed
and possible resolutions that lay ahead in the future discussed.
Key words: Stars: neutron – X-rays: bursts, binaries – Nuclear reactions

1. Introduction
Since 1969, ’type-I’ X-ray bursts are being detected fromspace-borne observatories between roughly 1 and 10 keV(Belian et al. 1972; Grindlay et al. 1976; Matsuoka et al.
1980; Makishima et al. 1981; Gottwald et al. 1986; Lewin
et al. 1993; Strohmayer & Bildsten 2006; Galloway et al.
2008). These bursts are due to thermonuclear shell
flashes of hydrogen and helium in the freshly accreted
upper layers of neutron stars in low-mass X-ray binaries (LMXBs; Hansen & van Horn 1975; Lamb & Lamb
1978; Wallace & Woosley 1981; Fujimoto et al. 1981), likethose on white dwarfs are responsible for classical novae.
The durations of most type-I bursts are between a few
seconds and a few minutes. The time profile of the X-ray
emission is typically a fast rise of duration a few seconds
and an exponential-like decay. The shell flash occurs atlarge (≫ 1) optical but small linear depth (roughly 1m) below the photosphere and temperatures may riseto order 1 GK. By the time the heat wave reaches thephotosphere, the temperature is a few tens of MK andthe typical peak energy of the photon spectrum is 5-10
keV. The flash itself lasts a fraction of second, although
it may take a few seconds to engulf the whole neutronstar. The burst decay phase essentially is the coolingof the neutron star and actually does not follow an exponential function but a power law (e.g., Cumming &Macbeth 2004; in ’t Zand et al. 2014). The duration of
the cooling is primarily set by the amount of mass thatis heated up or, in other words, the column depth of theignition (usually between 108 and 109 g cm−2 ).
In the fall of 1996, the just launched two Wide Field
Cameras (WFCs; Jager et al. 1997) onboard the ItalianDutch BeppoSAX observatory (Boella et al. 1997) conducted a nine-day long observation of the Galactic center region. Thanks to the 40 × 40 square degrees fieldof view per camera, the WFCs could simultaneously observe about half the Galactic low-mass X-ray binary population. One of the LMXBs in the field of view was
4U 1735-44. Figure 1 shows the nine-day light curve
resulting from that observation. It includes a remarkable feature on August 22, 1996: a fast rise exponentiallike decay phenomenon with a duration of a few hours.
Cornelisse et al. (2000) found this, and the spectrum,very reminiscent of a type-I burst except for the duration which is 103 as long. They proposed this as thelongest thermonuclear burst ever observed. This marks
the discovery of superbursts.
The discovery gave rise to searches in archival datafrom WFC and the All-Sky Monitor (ASM) on RXTE,
which yielded 12 more very long bursts that were published between 2001 and 2004 (see Table 1). Wijnands(2001) introduced the term ’superburst’. Cumming &Bildsten (2001) and Strohmayer & Brown (2002) introduced the first explanation of the phenomenon as a thermonuclear shell flash fueled by carbon at a column depth∼ 103 times deeper than for type-I bursts.

Fig. 1. Nine-day long light curve of 4U 1735-44 as observed with BeppoSAX-WFC, with the first detection of a superburst on August22, 1996 (from Cornelisse et al. 2000)

In this paper, we briefly discuss the observational facts(§2.) and theoretical considerations (§3.) that define ourcurrent understanding of superbursts, summarize the importance of superburst research (§4.) and touch on future
prospects (§5.).

2. Observational overview
2.1. Catalog & recurrence time
Table 1 presents a list of all 26 superbursts and theircharacteristics that have been reported up to January
2017. The most recent one is reported in these proceedings (Iwakiri et al. 2016a, see also Iwakiri et al. 2016b). It
is from 4U 1705-44, a LMXB that was already predicted
to be a superburster a decade ago (in ’t Zand et al. 2003).
The 26 superbursts are emitted by 15 low-mass X-ray binaries that are also emitters of type-I bursts. Currently,
MAXI on the ISS is the most efficient superburst discovering machine. All six superbursts since 2011 werediscovered with this device.
Five sources have exhibited multiple superbursts.
They have high accretion rates except 4U 0614+09. The
range of superburst recurrence times is between 10 d(for GX 17+2) to 10.5 yr (for 4U 1820-30). The average recurrence time is 4 yr, but this should be taken
as an upper limit because there are data gaps. in ’t
Zand et al. (2003) perform a statistical analysis of therecurrence time and find 2 ± 1 yr. Keek et al. (2006)
determine on a source-by-source basis and on the basisof the BeppoSAX-WFC database a lower limit to therecurrence time of usually 2 months. These determinations can probably be improved upon considerably withthe much larger data sets that is now available throughfor instance INTEGRAL and MAXI.

2.2. Host binaries
While in the early years (2000-2004) host binaries ofsuperbursts were all found to be LMXBs that are persistently accreting at a level of at least 0.1 times the
Eddington limit, the picture changed as the data grew.
There are now 4 transients among the 15 superbursters,
and one system (SAX J1828.5-1037) with an unknown
nature, but for certain with a low long-term average accretion rate like the 4 transients. Of the 7 host binaries
with (tentatively) known orbital periods, 2 are ultracompact X-ray binaries (UCXBs), meaning that the composition of the donors, and therefore that on on the neutronstar, is strongly deficient in hydrogen (4U 1820-30 and
4U 0614+09).
There are a number of prolific bursters that are semipersistent and did not exhibit a superburst yet: 4U 172834, EXO 0748-676 (off since 2011), 4U 1702-429, 1E
1724.3045 (in Terzan 2), A 1742-294, 4U 1812-12, GS
1826-24 and Cyg X-2. While it is too early to derivea physical meaning of this, it is something to keep inmind. It may be related to the question of fuel accumulation for superbursts and ignition conditions. in ’t
Zand et al. (2003) did an investigation of the average
α parameter among a number of persistent bursters. α
is the ratio of the gravitational energy released by accretion since the last burst to the nuclear energy released through the present burst. It should be between
about 30 and 200 (Lewin et al. 1993). in ’t Zand et al.
(2003) found a clear distinction between superbursters
and non-superbursters. The former ones have a significantly higher α value (>∼ 1000).
2.3. Distinguishing superbursts from other long bursts
After the discovery of superbursts, another kind of thermonuclear X-ray burst was discovered that is also long
but generally not as long as superbursts: intermediate
duration bursts (in ’t Zand et al. 2005; Cumming et al.
2006), the qualification intermediate referring to a duration between that of type-I bursts and superbursts (seeFig. 2). Since this may incur uncertainty in the identification of short superbursts, we discuss this somewhat
more.
Most intermediate duration bursts are thought to result from the ignition of thick helium piles on cold neutron stars. Due to the low temperature, ignition isreached at higher pressure and larger column depths,
and the helium needs much more time to reach ignition(days to weeks instead of hours) and bursts last longer.
The thicker piles and the fast 3α helium-burning nuclear
reaction will usually result in very high nuclear powersthat easily surpass the Eddington limit and result in verystrong photospheric expansion. The low temperatures go
hand in hand with low accretion rates. All these circumstances are found in ultracompact X-ray binaries withorbital periods shorter than about one and a half hourTable 1. Catalog of 26 superbursts from 15 sources, detected up to January 2017.

SourceTransient?

4U 0614+09
4U 1254-69
4U 1608-52
4U 1636-53

4U 1705-44
KS 1731-260
4U 1735-44
GX 3+1
GX 17+2

EXO 1745-248
SAX J1747.0-2853
4U 1820-30

Porb
(min)

Date (Instr.)51?

2005-03-12(ASM)
2014-11-03(MAXI)
1999-01-09(WFC)
2005-05-05(ASM+HETE)
1996-06-19(ASM)
1997-07-13(ASM)
1999-05-26(ASM)
2001-02-22(PCA+ASM)
2016-10-22(MAXI)
1996-09-23(WFC)
1996-08-22(WFC)
1998-06-09(ASM)
1996-09-14(WFC)
1999-09-23(WFC)
1999-10-01(WFC)
2000-09-08(WFC)
2011-10-24(MAXI+BAT)
2011-02-13(JEMX+MAXI)
1999-09-09(PCA)2010-03-17(MAXI+ASM)∗
1997-02-28(WFC)
1999-08-09(ASM)
2008-10-14(ASM)
2011-12-06(MAXI)
2011-11-12(MAXI)
2013-07-20(MAXI)

236
y
228

∼
279
10d?

y
y
11

Ser X-1

SAX J1828.5-1037
Aql X-1
∗

?
y

1137

Onset
obs.?

y
y

y
y

y
y
y
y

Decay
Time
(hr)

Lpeak
(1038
erg s−1 )Ṁ(edd.)

Nearestburst (d)y(1012
g cm−2 )E(1017
erg g−1 )

Ref.‡

2.1
5.2
6.0
4.5
3.1
1.8
2
1.5
2.2
2.7
1.4
1.6
1.9
1.0
0.7
2.2
10
4.2
1
0.5
1.2
3.6
1.4
2.3
2.3
4.3

> 0.1
2.8
0.4
0.5
1.0
> 0.9
0.8
0.7
> 1.0
1.4
1.5
0.8
1.0
1.3
1.7
1.8
0.7
3
3.4
> 3.3
1.6
> 0.19> 0.260.9
0.7
1.0< 0.01-367/+19

0.2

5

0.13
0.03

-51/+125-57/+104+96-122/+68-27/+15-2/+23

2.7
2.8

1.5
1.6

0.48

2.6

-6/+34../+374-62/+94
+2
+10
+2
+12

1.0
1.3

1.9
2.6

0.6

1.8

1.0

>1

1

10

0.55

2.3

2.1

4

1,a
2
3, 4, b
5
6, c
7
8
9, 4
10
11, 4
12, 4, d
13
14, e
14
14, 4
14, 4
15, 16
17, 2
18, f
19
20, 4
7
7
10
21
2, g

0.1
0.25
0.2
1
1
1
1
< 0.01
0.1
0.1
0.15
0.2
0.15
0.13
0.21
< 0.010.1-711/+25-168/+167+1549-162/+34
+309
+55

+389

According to Serino et al. (2016), this is not a superburst. It is listed here, because we believe that the alternative, it being an intermediate duration burst, isless likely given the high accretion rate. ‡ References (numeric for superburst data, alphabetical for orbital period): 1 - Kuulkers et al. (2010), 2- Serino et al.
(2016), 3 - in ’t Zand et al. (2003), 4 - Cumming & Macbeth (2004), 5 - Keek et al. (2008), 6 - Wijnands (2001), 7 - Kuulkers (2009), 8 - Kuulkers et al. (2004),
9 - Strohmayer & Markwardt (2002), 10 - Iwakiri et al. (2016a), 11 - Kuulkers et al. (2002), 12 - Cornelisse et al. (2000), 13 - Kuulkers (2002), 14 - in ’t Zandet al. (2004), 15 - Altamirano et al. (2012), 16 - Serino et al. (2012), 17 - Chenevez et al. (2011), 18 - Strohmayer & Brown (2002), 19 - in ’t Zand et al. (2011),
20 - Cornelisse et al. (2002), 21 - Asada et al. (2011), a - Shahbaz et al. (2008), b - Motch et al. (1987), c - van Paradijs et al. (1990), d - Corbet et al. (1986), e Bandyopadhyay et al. (2002), f - Priedhorsky et al. (1986), g - Chevalier & Ilovaisky (1991)

superbursts

intermediate
durationburstsmixed H/He bursts ÎHe bursts Î

Fig. 2. Preliminary histogram of burst durations as determined frombursts in the MINBAR archive (Galloway et al., in prep.)(Nelson et al. 1986) and that is indeed what observations
show (in ’t Zand et al. 2007; in ’t Zand & Weinberg 2010).
Peng et al. (2007) predict a small regime in accretion
rate of H-rich systems (≈ 0.003 times Eddington) where
also long helium bursts can happen after long series ofpure H bursts. However, this is a very small range of allowed accretion rates and pure H-bursts have never been
detected yet. Nevertheless, there are sporadic reports ofintermediate duration bursts from H-rich systems (Degenaar et al. 2010; Chenevez et al. 2007).
Identifying superbursts, particularly when data coverage is sporadic, can be cumbersome. One should, for instance, be careful about long bursts that are discoveredfrom UCXBs at low mass accretion rates. These may
be intermediate duration bursts. In fact, in a few cases
superburst detections had to be revised, see the careful
evaluation by Serino et al. (2016) . Furthermore, in myopinion this qualifies the identifications as superbursts
in 4U 0614+09 and SAX J1818-1036 as less certain (seealso Kuulkers et al. 2010 for 4U 0614+09).
2.4. Peak luminosities
Roughly 20% of type-I bursts have peak luminosities
near the Eddington limit (Galloway et al. 2008): 2.0 ×1038 erg s−1 for H-rich atmospheres and 3.4×1038 erg s−1
for H-poor atmospheres. The situation is different forsuperbursts. All superbursts except the PCA one from
4U 1820-30 (Strohmayer & Brown 2002) and possible
the JEM-X burst of SAX J1747.0-2853 (Chenevez et al.
2011) are sub-Eddington. This immediately shows thatthe fuel layer is not burning completely.
2.5. Precursors
Most superbursts are discovered with low duty-cycle instruments, particularly the ASM on RXTE and MAXI

Fig. 3. PCA-measured light curves of two bursts from 4U 1820-30.
Top panel: first 30 s of the superburst. Bottom panel: a type-I
burst from the same source. From Strohmayer & Brown (2002).

on the ISS. These devices observe more than 80% of thesky every 90-min satellite orbit, but only for about 1 min.
Therefore, it is easy to detect superbursts because theygenerally last longer than 90-min, but it is also difficult
to catch the onset of superbursts. The onset has been
observed in 8 of the 26 superbursts. Interestingly, in eachof these cases the onset is marked by a short burst. This
is often called a precursor, but actually there is only one
case (the PCA superburst of 4U 1820-30) where there istruly a brief period without burst emission between the
precursor and the main burst.
Figure 3 shows the onsets of a superburst and an ordinary burst from 4U 1820-30, detected with the highthroughput PCA. The superburst onset is characterized
by the precursor (from 5-18 s) and the superburst (starting at 18 s), and the dips in both these bursts. The dip
in the precursor is very short (less than the time resolution of 81 s) and possibly not complete to the pre-burst
flux level. The dip in the superburst drops to below thepre-burst level. Both of these dips are consistent withphotospheric expansion (Lewin et al. 1984) with adiabatic cooling, whereby the cooling in the second dip is
so strong that the X-ray signal is lost (Keek 2012). The
drop to below the pre-burst level is due to the photosphere covering up the X-ray emitting part of the accretion disk (e.g., in ’t Zand & Weinberg 2010).

Alternatively, Weinberg & Bildsten (2007) attributethe first brief dip as a pause between a spike due to a
shock breakout and a prematurely ignited type-I purehelium burst. However, Keek & Heger (2011) found thatthe heat released by the fallback of the photosphere after
the shock breakout may be sufficient to ignite the type-I
burst and that this occurs on time scales much shorter(10−5 s) than the dip time scale. After analyzing thespectral evolution during the dip, Keek (2012) concluded
that it could be attributed to photospheric expansion.
Keek (2012) also found that the precursor is more energetic than ordinary type-I bursts and, therefore, cannot
be powered solely by the burning of accreted helium.
They attribute the additional energy to the shock heating, supporting numerical models that predict that superbursts result in a detonation (Weinberg et al. 2006)
and shock that generates enough heat or overpressure to
power the precursor. A similar onset is observed in theother superburst detected with the PCA.
2.6. Burst quenchingSome superbursters are prolific emitters of type-I bursts
at the time of the superburst, with recurrence times ofmerely a few hours. These include KS 1731-26 and 4U
1636-536. But it is noticeable that this emission of type-I
bursts is quenched for a considerable period of time after the superburst (Kuulkers et al. 2002; Cornelisse et al.
2002), namely about one month. Apparently, the superburst influences the nuclear burning for a considerably
longer time than when its emission is visible.
Measuring the duration of burst quenching is difficult.
One needs a substantially high duty cycle to detect typeI bursts. This is not easily accessible with regular all-sky
monitors. Quench time measurements are very interesting, though. They provide a clean means to observe thetransition between stable and unstable nuclear burning(see Keek et al. 2012).
3. Theory
As with type-I bursts, the general picture is clear of
how superbursts come about (a deep thermonuclear shell
flash), but there are two essential issues that need resolution.
3.1. Inferring basic physics parameters
Cumming & Macbeth (2004) and Cumming et al. 2006
showed that it is possible to infer from the light curve, inparticular the peak luminosity and the decay profile, theignition column depth y12 = y/1012 g cm−2 and energy
yield E17 = E/1017 erg g−1 of the heated matter. The
higher the energy yield is, the higher the peak luminosity(up to the Eddington limit). The deeper the ignition is,
the longer the duration. An example of a fitted superburst decay is shown in Fig. 4. Table 1 shows the valuesof these two parameters for many superbursts. Due toFig. 4. The decay of the superburst from 4U 1735-44 (see Fig. 1
fitted with a model parametrized with y12 and E17 (see text andTable 1). From Cumming et al. (2006).

the lack of onset coverage, it is often difficult to obtainreasonable constraints on y12 and E17 , particularly thelatter. Therefore, these numbers lack in many cases.
Keek et al. (2015) improved the light curve diagnostic
power by including the slope of the temperature relation with depth as a free parameter. This impacts inparticular the rise phase of the light curve.
The expected values for CNO burning and 3α process
are E17 = 64 and 15, respectively. This is 1 to 2 ordersof magnitude larger than the observed values for superbursts. The energy yield for nuclear burning of carbon
to iron-group elements is about 10. The superburst from
4U 1820-30 has the very same value.
3.2. What fuel is burning?
The longer duration points to a deeper ignition, a higher
density and a higher temperature than for type-I bursts.
This makes it easier to overcome the Coulomb barrierof heavier elements than hydrogen and helium. The
next most abundant element, certainly after the burningof hydrogen to helium through the CNO cycle and theburning of helium through the 3α process, seems carbon
and this is the fuel that was considered by Strohmayer& Brown (2002) and Cumming & Bildsten (2001).
The E17 values are more or less consistent with carbon burning. They range between 1.5 and 10. Thisimplies mass fractions of burnt carbon between 15 and100%. Part of the remaining composition are the heavy
isotopes needed for the right ignition depth (Cumming &Bildsten 2001) and whose photo-disintegration, furthermore, might contribute up to half the superburst energy,
possibly lowering the required carbon in the fuel (Schatz

et al. 2003).
3.3. Issue 1: how to obtain and maintain enough fuel?
If temperature becomes too high, the Coulomb barrierof carbon atoms is easier overcome by ambient protons
and alpha particles through the reactions 12 C(p,γ)13 N
and 12 C(α,γ)16 O, thus destroying the carbon. This caneasily happen during helium flashes since the carbon andhelium are in the same layer. Thus, helium flashes are
responsible for both the production and destruction ofcarbon. Generally, it is thought that the only mannerin which carbon destruction can be prevented is by preventing temperature to grow too large during 3α burning. This is only possible during stable helium burning. There is observational evidence for that, throughthe measurements of the burst α parameter (see §2.2.).
Also theoretically, improvements are made in the understanding. Stevens et al. (2014) proposes that the rpprocess consumes all protons before they get a chance tocapture on carbon. Keek & Heger (2016) introduces a
new regime of stable hydrogen burning that increases thetemperature in the H-depleted layer underneath somewhat, yielding higher 3α rates without going runaway.
3.4. Issue 2: how to ignite the fuel?
For diluted carbon mixtures, as suggested by the E17
values, it is difficult to reach ignition conditions. The
temperature at a depth of y12 ≈ 1 is simply not highenough to overcome the Coulomb barrier. This temperature is primarily set by a heat flow from the crust,
where pycnonuclear reactions and electron captures provide a heat source of Qb = 0.1 − 0.6 MeV/nucleon. The
power scales with the accretion rate. Cumming & Bildsten (2001) initially resolved this issue by proposing reduced conductivity of the ignition layer, thus reducing
the cooling of the layer. Then the crustal heating may
be enough to reach ignition temperatures, provided themass accretion rate is in excess of 0.1 times Eddington.
However, two recent findings aggravate the issue again.
First, the detection of superbursts from transient accretors, starting with 4U 1608-52 (Keek et al. 2008) andwith an extreme case of EXO 1745-248 (Serino et al.
2012; Altamirano et al. 2012). The accretion rate, averaged over the superburst recurrence time, is (much)
lower for transients so that crustal heating is accordingly
weaker. Second, recent calculations of neutrino coolingthrough the URCA process in the crust (Schatz et al.
2014) and the deep ocean (Deibel et al. 2016) show it tobe much more efficient than previously thought.
The solution may come from finding shallow heatingprocesses, for instance due to rotational mixing and thus
deeper CNO burning (Keek et al. 2009) or freeze out ofheavy elements at the bottom of the ocean that induces
convection which heats up the superburst layer (Medin& Cumming 2011), but as yet the issue of unreachable
carbon ignition conditions remains.
4. Why study superbursts?
As may be clear from above, studying superbursts is interesting in its own right, but it is also very useful for
addressing a wide variety of scientific questions, for instance: 1) They are related to the same nuclear process(explosive carbon burning) that is thought to be responsible for type Ia supernovae. Understanding the ignition
will improve our understanding of type Ia SN ignition;2) Since the ignition is close to the neutron star crust,superburst characteristics depend on the thermal properties of the outer crust and, thus, provide a diagnosticof that crust (e.g., Cumming et al. 2006); 3) Superbursts
are sensitive probes of the neutron star spin and binary
orbit, through the detection of transient ms oscillations
during superbursts (Strohmayer & Markwardt 2002); 4)
Superbursts can be used as seismology probes of the neutron star interior, through the detection of oscillations
other than due to the spin (Strohmayer & Mahmoodifar2014); 5) Superbursts can be used as probes of the accretion disk. They irradiate and heat up the accretion
disk which can be observed through reflection features inthe superburst spectrum (e.g., Ballantyne & Strohmayer
2004; Keek et al. 2014). This irradiation happens bya simple spectral shape (black body) and over a broad
range of tractable temperatures; 6) The cooling of theneutron star envelope after a superburst has an observationally convenient time scale (days) to relatively easyprobe ignition conditions of hydrogen and helium burning (Keek et al. 2012).
5. Future
Only 11 superbursts have an observational coverage thatis better than 10%, strongly affecting measurements ofpeak luminosities, quench times and onset profiles. Only
2 superbursts have been observed with an effective areaof significantly more than ∼100 cm2 , strongly affecting
sensitive measurements of the spectrum (e.g., Keek et al.
2014) and interesting variability such as burst oscillations (Strohmayer & Markwardt 2002). Although MAXI
and other instruments are very useful in constraining
superburst recurrence times and ignition depths, it isobvious that much is to be gained from measurementswith higher duty cycles and larger sensitivity. This may
be obtained through 1) all-sky monitors that have large
2sky coverage and reasonable effective area >∼ 100 cm in
the 1-10 keV band and 2) large X-ray telescopes withquick read-out times that spend substantial amountsof observing time on the population of potential superbursters (bursters with high α values) or can be quickly
brought on target after the onset of a superburst. One

concept platform where both of these types of instruments/observations are foreseen in a optimum manner is
LOFT (e.g., Feroci et al. 2016; in ’t Zand et al. 2015),
with spin off concepts eXTP (Zhang et al. 2016) and
Strobe-X (Wilson-Hodge et al. 2017). In the meantime, Astrosat (Agrawal 2006) and NICER (Gendreau
et al. 2012) are and will be valuable assets in the studyof superbursts if caught (see also Keek et al. 2016).
Acknowledgements. Laurens Keek and Motoko Serino
are thanked for useful comments on an early draft ofthis paper. This paper uses preliminary analysis resultsfrom the Multi- INstrument Burst ARchive (MINBAR),
which is supported under the Australian Academy of Sciences Scientific Visits to Europe program, and the Australian Research Councils Discovery Projects and Future
Fellowship funding schemes.
References
Agrawal, P. C. 2006, AdSR, 38, 2989
Altamirano, D., et al. 2012, MNRAS, 426, 927
Asada, M., et al. 2011, ATel, 3760
Ballantyne, D. R., Strohmayer, S. E. 2004, ApJL, 602,
L105
Belian, R. D., et al. 1972, ApJL, 171, L87
Boella, G., et al. 1997, A&As, 122
Chenevez, J., et al. 2011, ATel, 3183
Chenevez, J., et al. 2007, A&A, 469, L27
Chevalier, C., Ilovaisky, S.A. 1991, A&A, 251, L11
Corbet, R.H.D., et al. 1986, 222, 15P
Cornelisse, R., et al. 2000, A&A, 357, L21
Cornelisse, R., et al. 2002, A&A, 382, 174
Cumming, A., Bildsten, L. 2001, ApJL, 559, L127
Cumming, A., Macbeth, J. 2004, ApJL, 603, L37
Cumming, A., et al. 2006, ApJ, 646, 429
Degenaar, N., et al. 2010, MNRAS, 404, 1591
Deibel, A., et al. 2016, ApJ, 831, 13
Feroci, M., et al. 2016, in Proc. SPIE, Vol. 9905, , 99051R
Fujimoto, M. Y., et al. 1981, ApJ, 247, 267
Galloway, D. K., et al. 2008, ApJS, 179, 360
Gendreau, K. C., et al. 2012, in Proc. SPIE, Vol. 8443, ,844313
Gottwald, M., et al. 1986, ApJ, 308, 213
Grindlay, J., et al. 1976, ApJ, 205, L127
Hansen, C. J., Van Horn, H. M. 1975, ApJ, 195, 735
in ’t Zand, J., et al. 2011, ATel, 3625
in ’t Zand, J. J. M., et al. 2003, A&A, 411, L487
in ’t Zand, J. J. M., et al. 2004, A&A, 426, 257
in ’t Zand, J. J. M., et al. 2005, A&A, 441, 675
in ’t Zand, J. J. M., et al. 2007, A&A, 465, 953
in ’t Zand, J. J. M., Weinberg, N. N. 2010, A&A, 520,
A81
in ’t Zand, J. J. M., et al. 2014, A&A, 562, A16
in ’t Zand, J. J. M., et al. 2015, ArXiv:1501.02776
Iwakiri, W., et al. 2016a, these proceedings

Iwakiri, W., et al. 2016b, ATel, 9882
Jager, R., et al. 1997, A&As, 125, 557
Keek, L. 2012, ApJ, 756, 130
Keek, L., Heger, A. 2011, ApJ, 743, 189
Keek, L., Heger, A. 2016, MNRAS, 456, L11
Keek, L., et al. 2006, A&A, 455, 1031
Keek, L., et al. 2008, A&A, 479, 177
Keek, L., et al. 2009, A&A, 502, 871
Keek, L., et al. 2012, ApJ, 752, 150
Keek, L., et al. 2014, ApJL, 797, L23
Keek, L., et al. 2015, MNRAS, 454, 3559
Keek, L., et al. 2016, ApJ, 826, 79
Kuulkers, E. 2002, A&A, 383, L5
Kuulkers, E. 2009, ATel, 2140
Kuulkers, E., et al. 2002, A&A, 382, 503
Kuulkers, E., et al. 2004, in AIP Series, Vol. 714, X-ray
Timing 2003: Rossi and Beyond, ed. P. Kaaret, F. K.
Lamb, & J. H. Swank, 257–260
Kuulkers, E., et al. 2010, A&A, 514, A65
Lamb, D. Q., Lamb, F. K. 1978, ApJ, 220, 291
Lewin, W. H. G., et al. 1984, ApJL, 277, L57
Lewin, W. H. G., et al. 1993, SSR, 62, 223
Makishima, K., et al. 1981, ApJL, 247, L23
Matsuoka, M., et al. 1980, ApJL, 240, L137
Medin, Z., Cumming, A. 2011, ApJ, 730, 97
Motch, C., et al. 1987, ApJ, 313, 792
Nelson, L. A., et al. 1986, ApJ, 304, 231
Peng, F., et al. 2007, ApJ, 654, 1022
Priedhorsky, W., et al. 1986, IAUC, 4247
Bandyopadhyay, R.M., et al. 2002, ApJ, 570, 793
Schatz, H., et al. 2003, ApJL, 583, L87
Schatz, H., et al. 2014, Nat, 505, 62
Serino, M., et al. 2012, PASJ, 64, 91
Serino, M., et al. 2016, PASJ, 68, 95
Shabaz, T., et al. 2008, PASP, 120, 848
Stevens, J., et al. 2014, ApJ, 791, 106
Strohmayer, T., Bildsten, L. 2006, New viewsof thermonuclear bursts (Compact stellar X-ray
sources), 113–156
Strohmayer, T. E., Brown, E. 2002, ApJ, 566, 1045
Strohmayer, T. E., Markwardt, C. B. 2002, ApJ, 577,
337
Strohmayer, T. E., Mahmoodifar, S. 2014, ApJL, 793,
L38
Van Paradijs, J., et al. 1990, A&A, 234, 181
Wallace, R. K., Woosley, S. E. 1981, ApJS, 45, 389
Weinberg, N. N., Bildsten, L. 2007, ApJ, 670, 1291
Weinberg, N. N., et al. 2006, ApJL, 650, L119
Wijnands, R. 2001, ApJL, 554, L59
Wilson-Hodge, C. A., et al. 2017, AAS Meeting
Abstracts, 229:339.04
Zhang, S. N., et al. 2016, in Proc. SPIE, Vol. 9905, ,99051Q